MS analysis of alkaline permanganate oxidation products

Journal of Southeast Asian Earth Sciences,Vol. 5, Nos 1-4, pp. 53~0, 1991

0743-9547/91 $3.00+ 0.00 PergamonPresspie

Printed in Great Britain

Characteristics of kerogens from Recent marine and lacustrine sediments: GC/MS analysis of alkaline permanganate oxidation products RvosrlI ISHIWATARI,* SHIGEO MORINAGA,* SHUICHI YAMAMOTO* and TSUTOMU MACHII-IARA~" *Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Setagaya,Tokyo 158,Japan and tTechnology Research Center, Japan National Oil Corporation, Midorigaoka, Hamura-cho, Nishitama-gun, Tokyo 190-11, Japan Al~ract--Extensive studies have been carried out by many workers on sedimentary kerogens. However little is known of the details of the chemical structure of kerogens and of the relation between immature and mature kerogens on a molecular basis. The present authors have been studying young kerogens (kerogens in young sediments). This study aimed to determine the structural peculiarities of young kerogens from marine and lacustrine sediments. Kerogen samples were isolated from marine (Tanner Basin, offshore California) and freshwater lake (Lake Haruna, Japan) sediments. The kerogens belong to Type II or III. These kerogenswere oxidizedby alkaline permanganate and analyzed for their degradation products by GC/MS. The major degradation products are aliphatic normal ~t,to-dicarboxylicacids with carbon numbers of 4-14; aliphatic normal monocarboxylicacids with carbon numbers of 8-26, and benzene mono-, di- and tetracarboxylic acids. A marked differencebetween kerogens from two environmentswas observedin the distribution of aliphatic dicarboxylic acids: C4--Ct0acids are higher for marine kerogens than for lacustrine kerogens. This difference is probably due to the difference in the fatty acid composition of precursory materials (e.g. phytoplankton). These results indicate that the molecular structure of kerogens reflects generally the molecular composition of pro-cursory materials, and consequently the present alkaline KMnO4 oxidation method is useful for subtyping of kerogens.

INTRODUCTION

EXPERIMENTAL

RECENT studies have shown that kerogen, the insoluble organic matter dispersed in sedimentary rocks, is the most important precursor of petroleum hydrocarbons (e.g. Tissot and Welte 1984). It is believed that the oil-producing potential of kerogens depends strongly upon the nature of their precursors. Therefore, the elucidation of the chemical structure of kerogens present in various sedimentary environments and the clarification of the relation between immature and mature kerogens on a molecular basis are important for a better understanding of the formation process of petroleum hydrocarbons from kerogens under geothermal stress. The authors have been investigating the characterization of young kerogens (kerogens in young sediments) using various methods including alkaline permanganate oxidation (Ishiwatari 1969, 1973, Ishiwatari et al. 1986, Machihara and Ishiwatari 1980, 1981, 1983). These studies have revealed the presence of an aliphatic structure in young kerogens, which has the potential for producing petroleum hydrocarbons on heating. We have also recognized a significant difference between lacustrine and marine kerogens in terms of molecular composition. As a part of our systematic study, we conducted a detailed characterization of young kerogens from marine and lacustrine sediments by alkaline potassium permanganate oxidation.

Kerogen samples

Lipid-free kerogens were separated from both a Recent marine sediment collected from Tanner Basin about 140 km offshore California and a lacustrine sediment of Lake Haruna, essentially by the extraction procedure described previously (Ishiwatari et al. 1977). Lake H a r u n a is a representative mesotrophic lake in Japan (altitude 1084 m, area 1.23 km 2, maximum depth 13.0 m, volume 0.01 km3). The major source of organic matter in Lake Haruna sediment is a phytoplankton population (Ishiwatari et al. 1980). Alkaline potassium permanganate oxidation

Two sets of alkaline permanganate oxidation were conducted for both marine and lacustrine kerogen samples. For both sets of oxidation, 20-30 mg of powdered kerogen sample was taken in a l0 ml glass tube, mixed with l0 ml of 2% KMnO4 in 1% K O H aqueous solution. The glass tube was sealed and allowed to react at 60°C for 1 h with mechanical shaking. At the end of the reaction, sodium sulfite and sulfuric acid were added to the mixture to reduce the excess permanganate and manganese dioxide at below p H 1. For one set of oxidation, degradation products were extracted from the reaction mixture with ethylacetate. 53

R. ISHIWATARIet al.

54

After silylation by Silyl-8 (Pierce Chemical Co.) of the degradation products, they were analyzed by gas chromatography (GC) or gas chromatography-mass spectrometry (GC/MS). For another set of oxidation, oxidation products were extracted with both hexane and ethylacetate and methylated by diazomethane. The methylated degradation products were analyzed by gas chromatography (GC) or gas chromatography-mass spectrometry (GC/MS).

GC /MS analysis GC analysis was conducted using a Hewlett Packard Model 5880A gas chromatograph with FID. Separation was performed on a 0.3 mm i.d. x 25 m fused silica capillary column chemically bonded with 5% phenylmethyl silicon (UP-2). The column temperature was programmed from 50 to 300°C at 4°Cmin ~. The GC/MS analyses were conducted on a Hewlett Packard Model 5985 quadrupole mass spectrometer interfaced directly with a Hewlett Packard Model 5890 gas chromatograph and equipped with a 0.3 mm i.d. x 25 m fused silica capillary column chemically bonded with 5% phenylmethyl silicon (UP-2), or on a Finnigan Model INCOS 50 quadrupole mass spectrometer interfaced with a Varian Model 3400 gas chromatograph. A chromatographic column was the same as described above. The column temperature was programmed from 70 to 300°C at 6°Cmin -L, or first maintained at 60°C for 1 min, programmed from 60 to 120°C at 30°Cmin ~ and then to 310°C at 5°C rain-J

KMn04 degradation products TMS-esters of degradation products. At the first stage of this experiment, degradation products were extracted with ethylacetate at pH 1 without pre-extraction with n-hexane, and were analyzed for aliphatic and aromatic acids (as TMS-esters) by GC/MS. This procedure enabled us to estimate an accurate molecular distribution of the degradation products. Figure 1 gives the gas chromatograms of degradation products (TMS-esters). Normal C44215 ct,og-dicarboxylic acids are the most dominant among the degradation products, with minor amounts of normal Cm-C30 monocarboxylic acids (n-Ci6 and n-Cl8 dominant). Benzene polycarboxylic acids are significant or dominant components in the degradation products. n-Hexane extractable degradation products. Normal hexane extracts of KMnO4 degradation products consist predominantly of aliphatic monocarboxylic acids with higher molecular weights. Figure 2 shows the gas chromatograms (RIC: reconstructed ion current) of n-current extracts of degradation products (methyl esters). As shown in Figs 2 and 3, normal C~2-C3~ ~,og-dicarboxylic acids (as indicated in the m/z 98 plots in Fig. 3) as well as normal C~0-C32 monocarboxylic acids (the m/z 74 plots in Fig. 3) have been confirmed for both kerogen samples. CI4, C15 , CI6 , CI7 , CI9 and C20 isoprenoid acids were identified by mass spectra and mass fragmentography.

TANNER BASIN KEROGEN EX. 6: HOOCICH~hCOOH id: Cl-bICH2h4CODH

RESULTS AND DISCUSSION

Ba:(~L(COOH},'

g

Kerogen type Table 1 gives H/C and N/C ratios together with 6 ~3C values of the isolated kerogens. The carbon isotope ratios of the isolated kerogens are within the range of typical values for marine and freshwater organisms. Only atomic ratios of H/C and N/C were calculated for the kerogen samples, because they contained a large amount of inorganic materials even after HC1/HF and further 6 N HC1 treatment. Therefore, the type of kerogen based on H/C vs O/C ratios (Tissot and Welte 1984) was estimated for other sets of kerogen isolated from these sediments. Thus, H/C vs O/C ratios indicate that a kerogen from the Tanner Basin sediment belongs to Type II, while a kerogen from Lake Haruna sediment to Type II or III. Table 1. H/C and N/C ratios and 6 t3C of the isolated kerogens Atomic ratio 6 ~3C Kerogen sample H/C N/C (%o) Tanner Basin 1.58 0.068 - 21.4 Lake Haruna 0.98 0.039 - 25.5

II 12~3 B2 I%

IG'

13

14

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B',:(~COOHb

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1

,.e'2,

~, . _ _

Fig. 1. Gas chromatogramsof TMS-esters of KMnO4 degradation products (ethylacetateextract)of kerogensfrommarineand lacustrine sediments.

GC/MS analysis of kerogens

55

contamination 16

TANNER BASIN KEROGEN 1E

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14

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(MIN)

Fig. 2. Gas chromatograms of n-hexane extract (methyl esters) of KMnO 4 degradation products of kerogens from marine and lacustrine sediments. Arabic numerals indicate the carbon number of monocarboxylic acids, A, B and C indicate C30, C3~ and C32 hopanoid acids, respectively.

6,10,14-Trimethylpentadecan-2-one (C is isoprenoid ketone) was also present in the degradation products. Both kerogens produced hopanoid acids, C, H2~_~002, ranging from n = 30 to 32 (peaks A, B and C in Fig. 2). Ethylacetate extractable degradation products. Ethylacetate fraction of KMnO4 degradation products contains polar components. As shown in Fig. 4, these fractions from both kerogens essentially consist of C 4 - C I 3 ct,~o-dicarboxylic acids, benzene polycarboxylic acids. Benzoic acid and the analogous series of benzene di- to tetracarboxylic acids (benzene 1,2- and 1,3-dicarboxylic; benzene 1,2,3-, 1,2,4- and 1,3,5-tricarboxylic; benzene 1,2,4,5- and 1,2,3,5-tetracarboxylic) have been confirmed. In addition, various methyl and dimethyl homologs are present for the benzene di- and tetracarboxylic acids.

Comparison between marine and lacustrine kerogens in terms of degradation products Kerogen samples from both marine and lacustrine sediments gave essentially the same series of degradation products, as summarized in Table 2. The only differences were observed in the relative abundances of degradation products. Normal ct,o~-dicarboxylic acids. A striking difference between marine and lacustrine kerogens is seen for the distribution of C4--C~0 ~t,og-dicarboxylic acids, as clearly shown in Fig. I. For Tanner Basin (TB)-kerogen, the amount of dicarboxylic acid decreases smoothly with increasing molecular weight from C4 to Ct5 with a slight shoulder around C7-C8. For Lake Haruna (LH)-kerogen, on the other hand, the distribution of dicarboxylic

acids in the range of C4-C~0 shows a maximum around C7 and C8. A similar difference in the a,o~-dicarboxylic acid distribution has been observed also for other marine and lacustrine kerogens (Morinaga and Ishiwatari unpublished). The detailed description of the difference will be reported elsewhere (Morinaga and Ishiwatari unpublished). Another marked difference between TB- and LH-kerogens is seen in the distribution of C22--C28 ~,e)-dicarboxylic acids (Figs 2 and 3). LH-kerogen gives higher amounts of C22-C28 dicarboxylic acids than the TB-kerogen. Aliphatic monocarboxylic acids. A significant difference between TB- and LH-kerogens is also seen in the distribution of normal fatty acids (monocarboxylic acids). The abundances of monocarboxylic acids in the range of C:4--C30 is high for LH-kerogen compared with that for TB-kerogen (Figs 2 and 3). Benzenepolycarboxylic acids. Figures 5-7 plot RIC specific for mass fragments of respective benzenepolycarboxylic acid methyl esters in the degradation products of both kerogens. As shown in Figs 5-7, there is no marked difference between TB- and LH-kerogens in the molecular distribution of benzenepolycarboxylic acids (ratios of benzene monocarboxylic, dicarboxylic, tricarboxylic acids; ratios of isomers of respective benzenepolycarboxylic acids). The amount of benzenepolycarboxylic acids (the sum of the amounts of benzene mono-, di-, tri- and tetracarboxylic acids) relative to aliphatic acids (at,(9-dicarboxylic acids plus monocarboxylic acids with carbon numbers larger than 4), is slightly different between TBand LH-kerogens. In this connection, we have defined in

56

R. ISHIWATARIet al. 14

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a previous paper (Ishiwatari et al. 1985) the following apparent aromaticity (fox): re, =

total benzenepolycarboxylic acids total aliphatic acids + total benzenepolycarboxylic acids

The apparent aromaticity obtained by this method has been found to correlate well with the aromaticity determined by r3C NMR method (Ishiwatari et al. 1985). A relationship between aromaticity determined by ~3C NMR ( f a nmr) and that determined by KMnO4 oxidation (fo~) is expressed by the following equation: f anmr= 0.29 + 0.58 X fox

(r = 0.93; n = 29).

The calculation indicates that f]* is 0.08 for TB-kerogen and 0.13 for LH-kerogen, respectively. These values are significantly lower than those obtained for kerogens from various immature marine sediments: 0.24-0.31 for kerogens for Cretaceous black shales (Ishiwatari et al.

1985), and 0.23-0.39 for kerogens from Quaternary (the Nankai trough) sediments (Machihara 1985). Origin of the KMn04 degradation products (probable contributors to kerogen structure)

The high abundance of aliphatic C4-C30 ~t,o~-dicarboxylic acids and Cm-C30 monocarboxylic acids has been interpreted to be due to high lipid (fatty acids, fatty alcohols, etc.) contribution to kerogens (Ishiwatari and Machihara 1982, Ishiwatari et al. 1986). Among 0t,oJdicarboxylic acids, C¢-C6 dicarboxylic acids can be originated from melanoidin (reaction products of amino acids with carbohydrates)-derived structures. The difference in the C4-Cm ~t,co-dicarboxylic acid distribution between the marine (TB)- and lacustrine (LH)-kerogens is explained by the difference in the nature of precursory lipid materials. Polyunsaturated fatty acids in marine phytoplanktons (algae), which are

GC/MS analysis of kerogens

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Fig. 4. Gas chromatograms ofethylacetate extract (methyl esters) of KMnO4 degradation products of kerogens from marine and lacustrine sediments. Arabic numerals indicate the carbon number of normal ct,eJ-dicarboxylic acids.

Table 2. Summary of organic compounds identified in the KMnO4 degradation products of young kerogens Degradation product identified Carbon No. Ultimate precursor ~t,oJ-Dicarboxylic acids Monocarboxylic acids Isoprenoid acids Isoprenoid ketone Hopanoid acids Benzenepolycarboxylic acids Methylbenzenepolycarboxylic acids

4-31 7 32 14-17, 19, 20 18 30-42

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Fig. 5. GC/MS plots of ethylacetate extract (methyl esters) of KMnO4 degradation products of marine and lacustrine kerogens, m/z 163 is for benzene dicarboxylic acids (M-31), and m/z 177 for methylbenzene dicarboxylic acids (M-31), respectively.

58

R. ISHIWATARIet al. 758

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Fig. 6. GC/MS plots of ethylacetate extract (methyl esters) of KMnO4 degradation products of marine and lacustrine kerogens, m lz 221 is for benzene tricarboxylic acids (M-31), m/z 235 for methylbenzenetricarboxylicacids (M-31), and m/z 249 for dimethylbenzene tricarboxylic acids (M-31), respectively.

probably an important precursor of polymethylene chains in marine kerogen, have a structure which tends to give larger amounts of C4 and C5 diacids than those in freshwater algae, as suggested from Table 3 (Pohl et al. 1968). On the other hand, unsaturated fatty acids in freshwater phytoplanktons have generally a double bond at the A9 position which tends to produce larger amounts of Cv-C9 ~,co-dicarboxylic acids than that of C4 ~,co-dicarboxylic acid (Machihara and Ishiwatari 1983). The high abundance of mono- and dicarboxylic acids longer than C20 for LH-kerogen may be explained by a high contribution of higher molecular weight fatty acids or alcohols of higher plant origin to the chemical structure of this kerogen. Benzenepolycarboxylic acids in the degradation products of kerogens may be predominantly derived from a melanoidin- or carbohydrate-derived structure (Ishiwatari et al. 1986). We have confirmed generation by KMnO4 oxidation of both melanoidins and carbohydrate-derived pseudomelanoidins of benzenepolycarboxylic acids with similar isomeric distributions to those observed for both kerogens. Other precursors of benzenepolycarboxylic acid are not known.

Geochemical significance

An alkaline permanganate oxidation method has been used by many workers to characterize kerogen structures. Ambles et al. 0985) and Vitorovic et al. (1984, 1986) have conducted KMnO4 oxidative degradation of fossil kerogens (Types I, II and III) and revealed the differences in molecular composition of degradation products among those kerogens. They extracted the degradation products from acidified solution with ethylether and analyzed them for methyl esters of the degradation products. Since our method for KMnO4 oxidation and analysis of degradation products is somewhat different from that of the above authors, a direct comparison of the results of young kerogens with those reported for fossil kerogens is not possible. However, the molecular composition of ethylacetate extracts of the degradation products of young kerogens is roughly close to that of a fossil Type II kerogen (Toarcian shale, Paris Basin) reported by Vitorovic et al. (1986), suggesting a close relation between fossil and young kerogens in chemical structure.

59

GC/MS analysis of kerogens 995

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Fig. 7. GC/MS plots of ethylacetate extract (methyl esters) of KMnO4 degradation products of marine and lacustrine kerogens, m/z 279 is for benzene tetracarboxylic acids (M-31), m/z 293 for methylbenzene tetracarboxylic acids (M-31), and m/z 307 for dimethylbenzene tetracarboxylic acids, respectively.

The present study has revealed that the differentiation between marine kerogens of Type II and lacustrine (freshwater) kerogens of the same type is based on KMnO4 degradation products. In particular, the difference in C4-CI0 :t,o-dicarboxylic acid distribution between marine and lacustrine (freshwater) kerogens, which was first noticed by the authors, is useful for subtyping Type II kerogens. Table 3. Polyunsaturated fatty acids present in marine algae (after Pohl et al. 1968) Fatty acids

~t,oJ-Dicarboxylic acids Environments expected to be produced

A9 + 7--16:1 A6 + 4---16:2 A7,10,13--16:3 A4,7,10,13--16:4 A9--18:1 A9,12--18:2 A9,12,15--18:3 A6,9,12,15--18:4 A5,8,11,14--20:4 A5,8,11,14,17--20:5 A4,7,10,13,16,19--22: 6

Fresh,* seat Fresh, sea Fresh, sea Fresh, sea Fresh, sea Fresh, sea Fresh, sea (Fresh), sea Sea Sea Sea

* Fresh-water algae. "~Sea-water algae.

8,9 or 6,7 5,6 or 3,4 6,7 3,4 8,9 8,9 8,9 5,6 4,5 4,5 3,4

CONCLUSIONS The following conclusions may be drawn from our KMnO4 degradation (oxidation) study of young kerogens from marine and lacustrine (freshwater) sediments: (1) Major degradation products of both marine and lacustrine kerogens are aliphatic C4-C~5 ~,o~-dicarboxylic acids. Benzenepolycarboxylic acids and normal C~0--C30 monocarboxylic acids are minor components in the degradation products. Various isoprenoid acids and hopanoid acids are also present in the degradation products. (2) Significant differences between marine (Tanner Basin) and lacustrine (Lake Haruna) kerogens are observed in the molecular distribution of the following degradation products: ~t¢o-dicarboxylic acids, monocarboxylic acids. These differences may be due to the difference in the nature of lipid components between marine and freshwater phytoplanktons. These differences are useful for subtyping of Type II kerogens. (3) Benzenepolycarboxylic acids in the degradation products of both marine and lacustrine kerogens may originate from melanoidin- or carbohydrate-derived structure in kerogens.

60

R. ISHIWATARI et al.

Acknowledgement--This study was partly supported by a grant from the Technology Research Center, Japan National Oil Corporation.

REFERENCES Ambles, A., Djordjevic, M. and Vitorovic, D. 1985. Multistage alkaline permanganate degradation of a type III kerogen. Chem. Geol. 48, 305-312. Ishiwatari, R. 1969. An estimation of the aromaticity of a lake sediment humic acid by air oxidation and evaluation of it. Soil Sci. 107, 53 57. Ishiwatari, R. 1973. Chemical characterization of fractionated humic acids from lake and marine sediments. Chem. Geol. 12, 113 126. Ishiwatari, R., Ishiwatari, M., Rohrback, B. G. and Kaplan, I. R. 1977. Thermal alteration experiments on organic matter from recent marine sediments in relation to petroleum genesis. Geochim. cosmochim. Acta 41, 815-828. Ishiwatari, R. and Machihara, T. 1982. Algal lipids as a possible contributor to the polymethylene chains in kerogen. Geochim. cosmochim. Acta 46, 825 831. Ishiwatari, R., Morinaga, S. and Simoneit, B. 1985. Alkaline permanganate oxidation of kerogens from Cretaceous black shales thermally altered by diabase intrusion and laboratory simulations. Geochim. cosmochim. Acta 49, 1825 1835. Ishiwatari, R.. Morinaga, S., Yamamoto, S., Machihara, T., Rubinsztain, Y., loselis, P., Aizenshtat, Z. and lkan, R. 1986. A study of formation mechanism of sedimentary humic substances.

I. Characterization of synthetic humic substances (melanoidins) by alkaline potassium permanganate oxidation. Org. Geochem. 9, 11-23. Ishiwatari R., Ogura, K. and Horie, S. 1980. Organic geochemistry of a lacustrine sediment (Lake Haruna, Japan). Chem. Geol. 29, 261-280. Machihara, T. 1985. Characterization of insoluble organic matter in sediments from the Nankai Trough, Deep Sea Drilling Project, Leg 87A, pp. 891-896. Initial Reports DSDP 87, U.S. Govt Printing Office,Washington. Machihara, T. and Ishiwatari, R. 1980. Characterization of young kerogen in a lacustrine sediment by alkaline potassium permanganate oxidation. Geochem. J. 14, 279-288. Machihara, T. and Ishiwatari, R. 1981. Characteristics of insoluble organic matter in lake sediments as revealed by alkaline potassium permanganate oxidation. Verh. int. Verein. theor, angew. Limnol. 21, 244-247. Machihara, T. and Ishiwatari, R. 1983. Evaluation of alkaline permanganate oxidation method for the characterization of young kerogen. Org. Geochem. 5, I 11-I 19. Pohl, P., Wagner, H. and Passig, T. 1968. Inhaltsstoffe yon Algen--II. Uber die unterschiedliche fettsaurzuzammensetzung von slz- und suBwasseralgen. Phytochemistry 7, 1565-1572. Tissot, B. P. and Welte, D. H. 1984. Petroleum Formation and Occurrence, 2nd Edn. Springer, Berlin. Vitorovic, S., Ambles, A. and Djordjevic, M. 1986. Improvement of kerogen structural interpretations based on oxidation products isolated from aqueous solutions. Org. Geochem. 10, 1119-1126. Vitorovic, D., Djordjevic, M., Ambles, A. and Jacquesy, J. C. 1984. Multistage alkaline permanganate degradation of a type II kerogen. Org, Geochem. 5, 259-265.